This study focused on informal reasoning regarding socioscientific issues. It sought to explore how content knowledge influenced the negotiation and resolution of contentious and complex scenarios based on genetic engineering. Two hundred and sixty-nine students drawn from undergraduate natural science and nonnatural science courses completed a quantitative test of genetics concepts. Two subsets (n = 15 for each group) of the original sample representing divergent levels of content knowledge participated in individual interviews, during which they articulated positions, rationales